In the field of rotary heat processing equipment it is important to make a distinction between directly heated equipment, such as directly heated rotary kilns and dryers, and indirectly heated rotary calciners, dryers, and other indirectly heated configurations.
Directly heated kilns and dryers employ rotary cylinders in which a hot gas stream (which gives up heat) and a stream of solid particles (which receives heat) are simultaneously passed through the center of the cylinder which may or may not be refractory lined. The hot gas and the cold solid material come into direct contact, and heat is transferred from the hot gas to the cold material largely by the heat transfer processes known as convection heat transfer and by radiation heat transfer. Directly heated cylinders are normally supported in two riding rings which are located approximately one-fifth of the over-all cylinder length in from each end. This configuration minimizes fiber stresses in the cylinder.
An indirectly heated rotary calciner is a common item of chemical process equipment having wide usage in the heat treatment of tonnage quantities of a variety of materials.
In the design of all prior indirectly heated rotary calciners, life expectancy of the heated section of the rotating cylinder has been a problem due to excessive stress in the heated section of the cylinder.
Indirectly heated rotary calciners, in general, are characterized as consisting of a rotating cylinder with a center section and minimum extensions on each end thereof, and a furnace surrounding said center section which provides heat. The furnace surrounds the center section which is unsupported. Outwardly of this center section are riding rings and trunnion rolls for support of said riding rings. Driving means, such as a girt gear and spur pinion, rotate the cylinder. Stationary end breechings provide a means to introduce and to withdraw material from the rotating calciner cylinder.
By contrast with directly heated rotary equipment, indirectly heated rotary calciners employ a different heat transfer mechanism. The major portion of the length of the rotating cylinder is enclosed in a stationary furnace and the outside of the cylinder, within the confines of the furnace, is heated with a stream of hot combustion products generated by burners (or a burner) which are normally mounted on the furnace wall. This heat is then conducted through the wall of the cylinder and is transferred to the bed of solid particles within the cylinder by a combination of radiation and conduction heat transfer processes.
In the case of indirectly heated rotary calciners and dryers, which are normally supported in two riding rings, the riding rings are located near the extreme ends of the cylinder to allow as much of the length of the cylinder as possible to be enclosed within the furnace.
This invention relates to indirectly heated rotary calciners and dryers rather than to directly heated rotary kilns and dryers.
The design of calciners is not well known. Limited design information is published in such textbooks as "The Handbook of Mineral Dressing" by Arthur F. Taggert (John Wiley & Sons, Inc., New York), which contains a section directed to the subject of rotary calciners and rotary dryers. Another test, "Perry's Handbook of Chemistry," also contains limited information on indirectly heated rotary calciners.
Traditionally, directly heated rotary equipment configurations such as rotary kilns and rotary dryers, as opposed to indirectly heated rotary calciners and dryers, are supported by riding rings located at any advantageous points along their exterior surface since no external furnace restricts their location. When two riding rings are employed as support points, each is normally located approximately 20% of the overall cylinder length from each end of the cylinder since this support arrangement equalizes, and minimizes, the various tensile and compressive forces within the cylinder.
It would be possible to provide cylinder extensions beyond the riding ring supports of an indirectly heated rotary calciner cylinder, each equal to approximately 33 to 50% of the heated length of the cylinder, to reduce the stress in the heated portion of the cylinder by nearly 50%, and this is one embodiment of this invention. But to do so would be expensive in terms of the equipment cost, and the equipment would occupy more space than shorter, weighted cylinder extensions, which act as moment arms, and are described in a preferred embodiment of the invention.
The preferred embodiment of this invention places weighted, artificial extensions on each end of the rotary indirectly heated calciner cylinder for the reduction of fiber stress in the heated portion of the cylinder, resulting in extended life of the heated portion of the calciner, and the potential to operate the calciner at higher temperatures.
It is, therefore, the intent of this invention to utilize the proposed cylinder extensions as moment arms which can be precisely calculated and then can be substituted by short extensions of the cylinder, each equipped with "dead" weights to precisely equal the required moment arm. It is recognized that the moment arm created by incidental items, such as a girt gear or a superficial extension of the cylinder section employed to preheat or to cool the calciner product, must be considered in the calculation of these moment arms.
The cylinder material may be any cast or fabricated alloy, the life of which will be greatly extended by these changes.